The use of redirecting mirrors having several mirror faces, such as for laser processing, is known in a variety of forms, such redirecting mirrors being used for various purposes.
For example, JP 01271088 A describes a laser processing machine in which a phase difference between an outer and an inner portion of a laser beam is compensated for by changing the position of a movable inner mirror element of a redirecting mirror relative to an outer mirror element by means of a linearly movable mechanism provided for the purpose.
JP 2005314198 A discloses an apparatus in which, in order to optimize a laser cutting machine for glass cutting, the cross-sectional shape and the intensity distribution of the laser processing beam are adjusted by using a facet mirror having a plurality of mirror segments, the inclination of which is adjustable, or a variable-shape mirror in order to redirect the laser beam.
GB 1,433,563 also describes a laser processing machine for glass cutting in which a smaller mirror face is arranged upstream of a larger mirror face in the beam path of a laser beam. The two mirror faces can be driven by servo motors in order to redirect the laser beam onto two different focal points along a cutting line formed on the glass surface.
DE 10 2004 043 895 A1 describes a micro-processing method in which a plurality of beam steering elements, which can be positioned independently of one another, direct laser radiation received in a beam steering element arrangement onto selectable sites on a substrate. A number of laser beam focusing modules are associated with the beam steering elements in order to focus each partial beam on the substrate.
DE 102 20 324 A1 describes a concave mirror as a pupillary filter for a catadioptric projection lens, this mirror being arranged in the region of a pupillary face of the lens. The concave mirror is divided into a number of annular or honeycombed mirror segments which are movable independently of one another and relative to one another by means of piezo-electric drive elements. The mirror can be used as a phase-shifting pupillary filter, it being possible to adjust the filter function by displacing the mirror elements relative to one another by raising or lowering those mirror elements by means of piezo elements.
As a rule, the use of segmented redirecting mirrors in laser beam welding serves to produce a larger vapor channel (keyhole), as a result of which a larger melt bath volume is formed. The enlargement of the vapor channel can be effected, for example, by double-focus or tandem welding in which the laser beam is split, for example, by a roof mirror, into two partial beams which are then focused on two focal points lying close to one another. This enables the gases formed during the welding process to escape better at the surface, and therefore weak points in the weld seam caused by pore formation are minimized.
An optical system for double-focus welding has become known, for example, from EP 0 823 304 A1. In that document, the laser beam is split at a redirecting mirror into a plurality of partial beams having different optical axes. In one embodiment, the redirecting mirror is made up of an outer and an inner mirror element, the outer mirror element having a cylindrical central drilled hole into which the inner mirror element is fitted so that the inner and outer mirror elements are axially displaceable and rotatable relative to each other, as a result of which it is possible to adjust the relative position of a focal point, associated with the first or second partial beam, on two workpieces to be joined together. After adjusting the desired relative position of the focal points, the adjustment is set securely by fixing the mirror elements in position relative to each other.
A disadvantage of enlarging the vapor channel by double-focus welding lies in the required larger energy input per unit length compared with laser welding with only one focus. It is known from electron beam welding to swing the beam transversely to the weld joint at high frequency, that is to say, at frequencies of more than 2000 Hz, and at a pendulum amplitude of less than one millimeter, by means of a pendulum mirror. Also in one embodiment in EP 0 823 304 A1, a further mirror is provided for redirecting the partial beams, that mirror performing an oscillating movement so that the two focal points can perform a pendulum movement on the workpieces in a direction perpendicular to the welding line.
U.S. Pat. No. 5,690,845 also describes an optical arrangement for laser processing which has a first redirecting mirror as a device for splitting a laser beam into a plurality of partial beams. A second redirecting mirror, separated from the first, focuses the partial beams on a plurality of focal points. In one embodiment, the first redirecting mirror has two or more flat mirror portions which can be tilted independently of one another. In a further, alternative embodiment, the first redirecting mirror has a plurality of concave, convex or flat mirror faces arranged concentrically around the optical axis of the laser beam. The first redirecting mirror can in this case be tilted or rotated, in each case as a whole, in a predetermined direction in order to produce a pendulum or spiral movement of the focal points on the workpiece.
The dynamic redirection and movement of the laser beam in order to produce the pendulum movement is usually effected—as in the above examples—with the aid of so-called scanner mirrors, in the case of which the entire mirror face is rotated about an axis by means of a galvanometer drive. If such a mirror is to be used at a site in the beam path at which the laser beam has a large diameter, large, and therefore heavy, mirrors are required for the purpose. This limits the maximum possible pendulum frequency to values <<1500 Hz. Smaller mirrors, and therefore higher frequencies, are possible only if they are arranged in the convergent or divergent portion of a beam caustic in the vicinity of the beam waist, such as, for example, in systems having an intermediate focus, or in the vicinity of the end of an optical-fiber cable.